1. Field of the Invention
The present invention relates to fluid dynamic bearing motors. More specifically, the present invention pertains to fluid dynamic bearing motors such as are used to support and rotationally drive one or more memory discs.
2. Description of the Related Art
The computer industry employs magnetic discs for the purpose of storing information. This information may be stored and later retrieved using a disc drive system. Computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs are typically circular in shape (though other shapes are known), and are comprised of concentric, or sometimes spiraled, memory tracks. Each track contains magnetic data. Transitions in the magnetic data are sensed by a magnetic transducer known as a read/write head. The transducer is part of the disc drive system, and moves radially over the surface of the disc to read and/or write magnetic data.
FIG. 1 presents a perspective view of magnetic media 10 as are commonly employed for information storage. In this view, a plurality of stacked magnetic discs 10′ is shown. The discs 10′ in FIG. 1 are shown in vertical alignment as is common within a disc drive system. Each disc 10 has a central concentric opening 5 for receiving a spindle (shown at 51 in FIG. 2). A rotary motor drives the spindle 51, causing the discs 10 of the disc pack 10′ to rotate in unison.
In operation, information stored in the magnetic layer of the disc 10 is read by a magnetic head assembly. The magnetic head assembly is part of a disc drive system, such as the system 50 shown in FIG. 2. FIG. 2 presents a top view of an exemplary disc drive system 50, with the magnetic head assembly seen at 58. The disc drive assembly 50 includes a servo spindle 52 and an actuator arm 54. The servo spindle 52 is motorized to pivot about an axis 40. More specifically, the servo spindle 52 is selectively positioned by a voice coil motor 57 which pivots the actuator arm 54, causing the arm 54 to move through arc 42. In this manner, the arm 54 can be positioned over any radial location “R” along the rotating disc surface.
The actuator arm 54 carries a flexure arm or “suspension arm” 56. The suspension arm 56, in turn, supports the magnetic head assembly 58 adjacent a surface of a disc 10. The head assembly 58 defines a transducer that is capable of reading magnetic information from the magnetic layer of the disc 10, or writing additional information on a reserved portion of the disc 10. The magnetic head 58 is typically placed on a small ceramic block, also referred to as a slider. The slider is aerodynamically designed so that it “flies” over the disc 10 as the disc is rotated at a high rate of speed.
As noted, the disc 10 itself is supported on a drive spindle 51. The drive spindle 51 rotates the disc 10 relative to the magnetic head assembly 58. FIG. 3 provides a perspective view of a disc drive assembly 50. In this arrangement, a plurality of discs 10′ are stacked vertically within the assembly 50, permitting additional data to be stored, read and written. The drive spindle 51 receives the central openings 5 of the respective discs 10. Separate suspension arms 56 and corresponding magnetic head assemblies 58 reside above each of the discs 10. The assembly 50 includes a cover 30 and an intermediate seal 32 for providing an air-tight system. The seal 32 and cover 30 are shown exploded away from the disc stack 10′ for clarity.
In operation, the discs 10 are rotated at high speeds about axis 45 (seen in FIG. 2). As the discs 10 rotate, the air bearing slider on the head 58 causes the magnetic head 58 to be suspended relative to the rotating disc 10. The flying height of the magnetic head assembly 58 above the disc 10 is a function of the speed of rotation of the disc 10, the aerodynamic lift properties of the slider along the magnetic head assembly 58 and, in some arrangements, a biasing spring tension in the suspension arm 56.
Each disc 10 has a landing zone 11 where the magnetic head assembly 58 lands and rests when the disc drive 50 is turned off. When the disc drive assembly 50 is turned on, the magnetic head 58 “takes off” from the landing zone 11. Each disc 10 also has a data zone 17 where the magnetic head 58 flies to magnetically store or read data.
As noted, the servo spindle 52 pivots about pivot axis 40. As the servo spindle 52 pivots, the magnetic head assembly 58 mounted at the tip of its suspension arm 56 swings through arc 42. This pivoting motion allows the magnetic head 58 to change track positions on the disc 10. The ability of the magnetic head 58 to move along the surface of the disc 10 allows it to read data residing in tracks along the magnetic layer 15 of the disc. Each read/write head 58 generates or senses electromagnetic fields or magnetic encodings in the tracks of the magnetic disc as areas of magnetic flux. The presence or absence of flux reversals in the electromagnetic fields represents the data stored on the disc.
In order to accomplish the needed rotation of discs, an electric motor is provided. The electric motor is commonly referred to as a “spindle motor” by virtue of the drive spindle 51, or “hub,” that closely receives the central opening 5 of a disc 10. FIG. 4 illustrates the basic elements of a known spindle motor design, in cross-section. The motor 400 first comprises a hub 410. The hub 410 includes an outer radial shoulder 412 for receiving a disc (not shown in FIG. 4). The hub 410 also includes an inner shaft 414. In this arrangement, the shaft 414 resides and rotates on a stable counterplate 440. A sleeve 420 is provided along the outer diameter of the shaft 414 to provide lateral support to the shaft 414 while it is rotated.
It can be seen that a bearing surface 422, or “journal surface,” is formed between the shaft 410 and the surrounding sleeve 420. In early arrangements, one or more ball bearing systems (not shown was incorporated into the hub 410 to aid in rotation. Typically, one of the bearings would be located near the top of the shaft, and the other near the bottom. A raceway would be formed in either the shaft or the sleeve for holding the plurality of ball bearings. The bearings, in turn, would be lubricated by grease or oil. However, various shortcomings were realized from the mechanical bearing system, particularly as the dimensions of the spindle motor and the disc tracks became smaller. In this respect, mechanical bearings are not always scaleable to smaller dimensions. More significantly, in some conditions ball bearings generate unwanted vibrations in the motor assembly, causing the read/write head to become misaligned over the tracks. Still further, there is potential for leakage of grease or oil into the atmosphere of the disc drive, or outgassing of the components into this atmosphere.
In response to these problems, hydrodynamic bearing spindle systems have been developed. In these types of systems, lubricating fluid is placed along bearing surfaces defined around the rotating spindle/hub. The fluid may be in the form of gas, such as air. Air is popular because it avoids the potential for outgassing of contaminants into the sealed area of the head disc housing. However, air cannot provide the lubricating qualities of oil or the load capacity. Further, its low viscosity requires smaller bearing gaps and, therefore, higher tolerance standards to achieve similar dynamic performance. As an alternative, fluid in liquid form has been used. Examples include oil and ferro-magnetic fluids. A drawback to the use of liquid is that the liquid lubricant should be sealed within the bearing to avoid leakage. Any loss in fluid volume results in a reduced bearing load capacity and life for the motor. In this respect, the physical surfaces of the spindle and of the housing would come into contact with one another, leading to accelerated wear and eventual failure of the bearing system.
Returning back to FIG. 4, the motor 400 of FIG. 4 represents a hydrodynamic bearing system. A thrust plate 430 is disposed between the shaft 414 and the surrounding sleeve 420. Fluid is injected in gaps maintained between the shaft 414 and surrounding parts, e.g., the counterplate 440, the sleeve 420, and the thrust plate 430. The fluid defines a thin fluid film that cushions relative movement of hub parts.
The motor 400 is actuated by energizing coils in a stator in cooperation with one or more magnets. In the view of FIG. 4, magnets 450 are seen disposed within the hub 410, while stator coils 452 are provided on a base 460. The magnets 450 and stator coils 452 interact to provide rotational movement of the hub 410.
Additional details of fluid dynamic bearing systems are provided in U.S. patent application Ser. No. 10/099,205 filed Mar. 13, 2002, and entitled “Low Power Fluid Dynamic Bearing.” That application is commonly owned with the present application, and is incorporated herein in its entirety by reference. Of interest, that application presents various hydrodynamic motor designs wherein a thrust plate 430 is not employed.
As noted, it is important to retain fluid within the bearing surfaces for a hydrodynamically operated spindle motor. Various architectures have been proposed for retaining fluid within the bearing surfaces. Certain patents present a mechanical seal. For example, U.S. Pat. No. 5,347,189 entitled “Spindle Motor with Labyrinth Sealed Bearing” provides a labyrinth seal outside one of the bearings. The labyrinth seal has two parts that mate to form a tortuous flow path for fluids. This serves to inhibit the escape of grease from ball bearings. U.S. Pat. No. 5,925,955 entitled “Labyrinth Seal System” provides an alternative seal system for an electronic spindle motor.
Other patents provide for a grooved pattern that serves to retain fluid within a spindle motor. U.S. Pat. No. 6,149,159 entitled “High Pressure Boundary Seal” provides for a “herringbone pattern” of grooves along or adjacent the outer surface of the shaft. A zone of high pressure is created at or about the center of the pattern, thereby creating a high pressure boundary seal. This, in turn, prevents the flow of lubricating fluid from the interior of the motor or the bearing into the interior section of the disc drive housing. Another example is U.S. Pat. No. 5,533,812 entitled “Single Plate Hydrodynamic Bearing with Self-Balanced Fluid Level,” which offers a thrust plate having grooved surfaces.
Still another means for retaining fluid within a hydrodynamically operated bearing surface for a spindle motor is presented in U.S. Pat. No. 5,524,986. This patent is entitled “Fluid Retention Principles for Hydrodynamic Bearings.” A flexible membrane is provided at one end of the fluid gap. The spring force of the membrane allows the gap volume to adjust with fluid changes as temperature fluctuates. In this respect, the membrane is flexible, and absorbs any increase in volume of the bearing fluid. The '986 patent also introduces the principle of a capillary seal. In this respect, a capillary seal is provided at one end of the gap. The capillary seal design helps retain a volume of lubricant oil within the system necessary for continuous motor operation.
One problem presented with the capillary seal design is that an end of the bearing gap is exposed to the ambient environment of the disc drive housing. This, in turn, can lead to a slow but progressive oil loss by evaporation. The lubricant oil is selected to have a low vapor pressure to reduce evaporation. Nevertheless, over the life of the motor a noticeable amount of lubricant is lost from the capillary seal by evaporation, as well as from vapor diffusion in the gas phase.
To compensate for the oil loss, the capillary seal dimensions are designed to hold a larger amount of oil than would otherwise be necessary. However, the available reservoir volume is limited by geometrical size constraints and by requirements for seal splash robustness during shock events.
Thus, a need exists for an improved fluid dynamic bearing system for a spindle motor that retains liquid within and along the bearing surfaces. Further, there is a need for such a motor that minimizes oil loss due to evaporation. Still further, there is a need for such a motor that minimizes the amount of oil that is lost from the capillary seal over the life of the motor.